The present invention relates to a hammer drive apparatus for an impact printer comprising means for ensuring that all characters will be printed with equal density.
In a typical apparatus of the present type, a hammer is driven by an electromagnet to impact a type element against paper through a ribbon for printing.
A number of type elements for various characters are carried on the spokes of a daisy wheel or similar type element carrier which is rotated to a position for printing the selected character. The various characters have different printing areas. For example, the character "W" has a large printing area whereas the character "i" has a small printing area. If the printing hammer is driven with the same force for printing all characters, the density of the printed character "W" will be lower than the density of the printed character "i". The reason for this is that the impact force is distributed over a larger printing for characters such as "W" than for characters such as "i".
Various expedients have been proposed in the prior art in an attempt to print all characters with equal density. Japanese patent application disclosure No. 52-48926 teaches the construction of an example of the prior art. In this system, a high current is caused to flow through an electromagnetic coil which, when energized, generates an electromagnetic force which drives the hammer to impact the type element and paper. The initial high current causes the hammer to move quickly against static friction and inertia forces. Then, the current is attenuated over a predetermined period of time in accordance with a time constant corresponding to the printing area of the character. The attenuation rate is greater for characters of small printing area than for characters with large printing area so that the impact force is decreased for characters of small printing area and vice-versa.
Although this system is theoretically desirable, it is difficult and expensive to embody in actual practice. This is because accurate analog control of the attenuation of the hammer drive current requires an anlog switch, an operational amplifier, a variable current source and other components which must be constructed and adjusted to operate with a small rate of error. Such an analog control system is expensive and generally low in reliability.
Other known references in the art include Japanese utility model publication No. 52-22179 and Japanese patent application disclosure No. 49-123232. The former reference generally teaches how to drive a coil for a printing hammer in response to a high frequency pulse train. The coil is connected to a power source only when the drive pulses are logically high.
The latter reference discloses means for switching a hammer drive coil into connection with a power source and means for providing a low resistance path across the coil when the coil is disconnected from the power source so that current flow continues for a period of time due to counter electromotice force.
Although these references are relevant to the general field of art, they do not disclose the novel and advantageous combination of the present invention.